Rainfall seasonality and drought performance shape the distribution of tropical tree species in Ghana

Tree species distribution in lowland tropical forests is strongly associated with rainfall amount and distribution. Not only plant water availability, but also irradiance, soilfertility, and pest pressure covary along rainfall gradients. To assess the role of wateravailability in shaping species distribution, we carried out a reciprocal transplantingexperiment in gaps in a dry and a wet forest site in Ghana, using 2,670 seedlings of23 tree species belonging to three contrasting rainfall distributions groups (dry species, ubiquitous species, and wet species). We evaluated seasonal patterns in climaticconditions, seedling physiology and performance (survival and growth) over a 2-yearperiod and related seedling performance to species distribution along Ghana's rainfall gradient. The dry forest site had, compared to the wet forest, higher irradiance,and soil nutrient availability and experienced stronger atmospheric drought (2.0 vs.0.6 kPa vapor pressure deficit) and reduced soil water potential (-5.0 vs.-0.6 MPasoil water potential) during the dry season. In both forests, dry species showed significantly higher stomatal conductance and lower leaf water potential, than wet species, and in the dry forest, dry species also realized higher drought survival andgrowth rate than wet species. Dry species are therefore more drought tolerant, andunlike the wet forest species, they achieve a home advantage. Species drought performance in the dry forest relative to the wet forest significantly predicted speciesposition on the rainfall gradient in Ghana, indicating that the ability to grow and survive better in dry forests and during dry seasons may allow species to occur in lowrainfall areas. Drought is therefore an important environmental filter that influencesforest composition and dynamics. Currently, many tropical forests experience increase in frequency and intensity of droughts, and our results suggest that this maylead to reduction in tree productivity and shifts in species distribution.